A systematic review of resilient performance in defence and security settings

A narrative systematic literature review was conducted to explore resilient performance in defence and security settings. A search strategy was employed across a total of five databases, searching published articles from 2001 onwards that assessed performance and optimal function in relation to resilience, in defence and security personnel. Following narrative synthesis, studies were assessed for quality. Thirty-two articles met inclusion criteria across a range of performance domains, including, but not limited to, course selection, marksmanship, land navigation, and simulated captivity. Some of the key findings included measures of mental toughness, confidence, and a stress-is-enhancing mindset being positively associated with performance outcomes. There was mixed evidence for the predictive value of biomarkers, although there was some support for cortisol, dehydroepiandrosterone sulfate (DHEA-S) and neuropeptide-y (NPY), and vagal reactivity. Interventions to improve resilient performance were focused on mindfulness or general psychological skills, with effects generally clearer on cognitive tasks rather than direct performance outcomes in the field. In sum, no single measure, nor intervention was consistently associated with performance over a range of domains. To inform future work, findings from the present review have been used to develop a framework of resilient performance, with the aim to promote theoretically informed work.


Introduction
Defence and security personnel operate in Volatile, Uncertain, Complex, and Ambiguous (VUCA) settings. To perform well in VUCA settings, a range of physical, behavioural and psychological competencies are required. Examples include problem solving, being able to learn new roles, tasks and technologies, and demonstrating interpersonal, cultural and physical adaptability [1]. To help personnel maintain and optimise performance in these and other similar competency areas, it is important to understand the antecedents that explain variability in and processes seem to be captured by a constellation of biological, psychological and social dynamics. Indeed, exploring the biological, psychological, and social aspects of resilience draws on a longstanding tradition of studying stress and performance from a biopsychosocial basis [14,25,26]. Consistent with this biopsychosocial view, Kalisch, Cramer [27] recently proposed the concept of a dynamic resilience network, in which psychological resilience is a dynamic context specific construct, dependent on person-environment interactions. Conceptualised with a focus on mental health, resilience can be understood by the dynamics in the interconnections between physical, affective, cognitive, and social nodes. Stressor exposure can lead to deterioration in the different nodes, which if left unchecked can spread through the network. For example, deterioration in the physical node (e.g., sleep) can impact upon affective responses (e.g., increased negative mood), which might disrupt cognitive capability (e.g., lack of concentration) and damage social relations (e.g., being short-tempered with peers). Resilience would be demonstrated by quick recovery in the affected nodes and limited disruption through the network (e.g., maintaining a positive mood state). According to Kalisch, Cramer [27], so-called resilience factors (e.g., personality traits), act as moderators and protection to the network to foster resilient responding under conditions of stress. The model provides a coherent link between important resilient outcomes (i.e., the nodes), resilient factors (i.e., potential antecedents) and a consideration of these dynamics over time. This temporal view can be applied to resilient performance in VUCA settings. Chronic exposure to stress may need to be managed over extended periods to avoid adverse performance-related impacts. Whereas acute stress exposure may require personnel to transition quickly from periods of low activation to engagement in psychologically and physically demanding tasks. Resilience is likely to play an important role in all of these situations; which defence and security personnel encounter multiple times across their career lifespan.
Prior research studies conducted with defence, security, and law enforcement personnel document several measurable performance related metrics linked to resilient function, such as passing selection courses, marksmanship, and navigation [22,28,29]. Researchers have also examined the role of various individual difference factors including personality and motivation [30,31], psychological skills such as imagery and activation regulation [32], and biomarkers like cortisol, neuropeptide-Y (NPY), dehydroepiandrosterone (DHEA) and vagal tone [33], in an attempt to explain who is likely to respond resiliently to stressor exposure and able to maintain performance. Currently, these disparate yet related studies have not been synthesised in a way that might inform the biopsychosocial factors underpinning resilient performance and thus contribute to methods for monitoring and optimising the function of defence and security personnel. With this in mind, the present work builds on recent contributions by van der Meulen, van der Velden [13] and examines resilience with a specific focus on its relationship with performance (as opposed to mental health) in defence and security settings.
The overarching aim of the present work was to conduct a narrative systematic review of resilient performance in defence and security settings. The review was organised around core research questions which included: 1. What theories and models have been used to study factors associated with resilient performance in defence and security settings and other high performance domains? 4. What is the quality of papers published in the field?
The intended outcome of the review was to inform recommendations on the methods that could be used to measure resilient performance, to evaluate the effectiveness of related training programmes designed for defence and security personnel and to develop a framework of resilient performance for future research.

Methods
As part of the present review, research studies on the mechanisms, measures, and approaches used to evaluate and to train resilient performance in defence and security settings were identified and assessed. A focus was placed on understanding performance under conditions of stress (psychological and physical), with resilient performance demonstrated by an individual who is able to execute their competencies successfully under demanding conditions, such that situational performance is equal to, or exceeds, their usual performance. In contrast, an individual who is unable to execute their competencies successfully under demanding conditions, such that situational performance is below their usual standard, and not sufficient to complete the task demands successfully is thought to not be demonstrating resilient performance. Resilience was defined as the maintained or improved execution of competence under situational duress, and a resilience training environment was defined as one which aims to enhance or maintain resilient performance in a high-stress environment with a focus on biopsychosocial markers and targets. A high-stress environment was defined as those that are VUCA, where there is potential for harm and effort is required.

Literature search strategy and selection criteria
A systematic literature search for articles published in English from January 2001 to October 2020 was conducted, to identify primary research studies that have investigated the relationship between psychological resilience and performance in high-stress environments. Subsequently an updated search was performed, searching for articles published from October 2020 to January 2022, to maintain currency for the completed review. Target populations included military personnel, security/defence personnel, and those who embark upon expeditions or space travel. A pilot search was initially conducted including a broad target range of populations (e.g., elite sports people, first responders). Based on this search the heterogeneity of papers was such that we delineated the search terms to the target populations of military personnel, security/defence personnel, expeditions/space travel to ensure a more focused and relevant population for our literature review. The decision to include articles published from 2001 onwards was based on the timing of when psychological resilience, beyond the broader study of stress, emerged as a specific focus in scientific research, and a contemporary view of the military environment, which has been substantially shaped by operations that have taken place since 2001 [34,35].
Search strategy. In accordance with the PRISMA guidelines [36] the search strategy was developed for this systematic review and the search was conducted (Commenced 6 th of August, 2020). Electronic literature searches were carried out using the title field in the following online databases: MEDLINE via PubMed, PsychINFO, Scopus, and Web of Science. Grey literature was searched for in the Defence Technology Information Centre (DTIC). Reference lists of included studies and relevant review articles identified through the search were also checked for relevant articles, including military reports. The search terms used were: (Resilien � OR stress � OR cope � OR "mental toughness" OR challenge OR threat OR pressure OR risk OR fortitude OR thriv � OR flourish � OR grit OR hardiness OR robust � ) AND (Perform � OR behavio � OR response � OR biopsychosocial OR psych � OR intervention OR program OR training OR develop � OR accuracy) AND (Military OR army OR airforce OR navy OR "defence personnel" OR "defence staff" OR "defence employees" OR "SOF" OR "special forces" OR "security personnel" OR "security staff" OR "security employees" OR space OR expedition OR police).
Inclusion and Exclusion criteria: Articles were identified that met the following criteria: original, peer-reviewed studies including the following designs: theoretical/conceptual papers, cohort studies, cross-sectional studies, case-control studies, qualitative studies, and prospective and longitudinal designs. Articles were required to be published in English, from 2001 onwards, and relate to performance and optimal function in relation to resilience. Articles were excluded if they focused on participants aged 18 and under, or undergoing basic and entry level training programmes in defence and security environments. Pre-search exclusion criteria also included wrong population, wrong paper type such as review articles, editorials, conference abstracts, book chapters, not published in English, and papers that did not measure performance.
Data extraction. Paper selection process. Titles and articles retrieved by the online search strategy were reviewed by a member of the research team. If titles appeared to meet the inclusion criteria citations were sent to an online reference manager to be stored for abstract review; if it was not clear from the title whether the article subject matter was of interest a pre-screen of the abstract was performed. Duplicates of papers retrieved were then removed and abstracts of the remaining papers were reviewed against the inclusion and exclusion criteria by the two members of the research team. Full texts were obtained for the final selected articles. See Data extraction. Following the search strategy, papers identified for further review were read by at least two members of the research team, and every paper was considered for quality and relevance by the research team as a whole during meetings. Data were extracted from eligible articles and entered into a pre-defined data-extraction sheet. The following data were extracted from each article: paper details (authors, year), study design (sample size, participant details such as age and area of occupation, recruitment method and procedure), details of the measure of performance, theoretical approach, markers of resilience, details of the training program (if relevant), key findings. Inconsistencies were resolved in consensus meetings attended by the whole project team.
Literature search outcome. The original literature search yielded 3047 articles. 1546 duplicates were deleted, and 1415 were discarded as irrelevant/ineligible based on titles and abstracts. The remaining 131 articles were assessed for inclusion by a member of the research team using the full text, and a further 99 were excluded. This resulted in 32 articles that met inclusion criteria for the systematic review. The updated search retrieved a total of 415 titles, after which 274 remained, following duplicate removal. Following this 254 were discarded as irrelevant or ineligible leaving 20 titles being assessed for full inclusion, however none met the full criteria for inclusion in the review. Due to the heterogeneity of papers and small numbers of articles retrieved to individual categories the use of meta-analysis techniques was deemed not appropriate for the current review, therefore a solely narrative approach was applied to the current systematic review.
Assessing study quality and relevance. For each included article, two members of the research team evaluated the study quality using the Mixed Methods Appraisal Tool (MMAT), a tool designed for the appraisal stage of systematic mixed studies reviews [37]. The MMAT uses two initial screening questions, and a further five items to determine quality, and yields a score of 0 to 5, where higher scores indicate higher quality research. Members of the research team met to reach consensus on ratings and the papers' applicability to the review based on the inclusion and exclusion criteria.

Overview and quality of included studies
The search retrieved 32 that satisfied inclusion criteria for data extraction and narrative synthesis, with one paper retrieved comprising of three individual studies, making a total of 34 studies included in the review. Studies were conducted internationally, with a range of defence and security personnel. A summary of the included papers, including country or origin and sample details and extraction of study characteristics is presented in Table 1. Out of the included studies [34] the majority were awarded a quality rating of three (56%) while 32% were awarded a higher rating of four. Lower proportions of paper were awarded the lower   ) [43]. Thriving under adversity (Epel 1998) [44]. Weinberg, 2010 [50,51]).

Mental toughness-Mental
Toughness Inventory (developed in study 2) Psychological hardiness-15-item Norwegian DRS Self efficacy-8-item new general self-efficacy scale.
Physiological N/A Mental toughness significantly predicted selection test outcome, even when hardiness and selfefficacy were considered. That is, those participants who self-reported as more mentally tough were more likely to pass selection. The specific facets of commitment, control, challenge and self-efficacy did not predict performance outcome.

Experimental (randomised)
US active-duty soldier cohorts were randomly assigned by unit to either the 8-hour MMFT variant that emphasized didactic content (M8D) or the 8-hour MMFT variant that emphasized MT practices (M8T). All participants were tested before (T1) and after (T2) an 8-week training period.
No specific theory of resilience applied. Psychological At T2, both MT groups had higher A' scores and selfreported being significantly (M8D) or marginally (M8T) more 'aware' of their attention compared to NTC.
In addition, a direct comparison of the two MT groups revealed that M8T had higher A' and lower ICV relative to M8D, suggesting fewer attentional lapses and greater attentional stability in the M8T group. As predicted, CIV A' scores remained stable over time while the NTC group's scores significantly declined from T1 to T2. Like NTC, M8D also significantly degraded over time, suggesting that participating in this MT course did not sufficiently protect against performance costs over time. In contrast, M8T's A' scores remained stable from T1 to T2. These results suggest that attentional performance, which degrades over the predeployment interval, is better protected by M8T vs. M8D. A fourth company was assigned to NTC group, nonrandomized. Soldiers were assessed in the week before the training period (week 0, T1), following the training period (week 5, T2) and 4-weeks following T2 (week 10, T3).

SART Working Memory Delayed-Recognition Task with Affective Distracters
No specific theory of resilience applied. Psychological While task performance declined over the highdemand military field training interval for all participants, the PE group showed less decline when compared to the NTC group, as well as the ME group. PE group was estimated to decline less compared to the NTC group by 0.08 units of A 0 in the SART from T1 to T2, which represent roughly eight fewer missed SART target trials. Only directional but not significant differences between the PE and NTC groups in the amount of attentional change from T1 to T3, suggesting that the attentional benefits were not completely maintained across the entire study interval (10-weeks in total). PE vs ME comparison on SART was comparable. PE group was estimated to decline less compared to the NTC group by 3.07% accuracy in the WMDA task from T1 to T3, which represents roughly two fewer incorrect WMDA trials on average. these results suggest that not only was PEdelivered MBAT protective against decline relative to the NTC group, but it was also protective relative to MEdelivered MBAT. Effects of acute cold stress were observed for the visual vigilance cognitive testcorrect hits and reaction time were impaired by cold stress.
There were also effects of acute cold stress in impairing the pattern recognition test, and on increasing the tension mood state. The DHEA-S-cortisol ratio was significantly and positively associated with performance during the interrogation task. There was also a negative correlation between the stress-induced levels of salivary cortisol and interrogation performance, indicating that increased salivary cortisol was associated with poorer performance. Additionally, there was a negative association between stress-induced dissociation scores and interrogation performance (r = -0.51, p<0.01), indicating that those with fewer dissociative symptoms performed better. Decreased baseline vagal tone predicted superior performance in a highly stressful underwater navigation exam at the end of the course. Heart rate failed to show any significant effect on the performance variables or the predictive power of HF spectral power. There was no effect of vagal tone on the written exam. ratings of two (3%) and the highest rating of five (9%). Ratings indicate that the majority of papers had moderate quality, suggesting improvements could be made in future research to address this.

(Continued )
Psychological theories and models used to study resilience in defence and security settings. The collation of theories and models from the included papers is challenging for various reasons. The majority of studies retrieved did not articulate explicit underpinning, or overarching, resilience theory in the development of study rationale. Therefore, the included studies offer little by way of theoretical consensus. For the small selection of papers that did include a theoretical underpinning [22,40,48,73], theories included the maintenance of equilibrium under adversity [41,74], stress and coping [75], thriving under adversity [44] and psychological hardiness [38]. The result is surprising given the availability of resilience theory in the general and performance-specific literature, such as the compensatory model (resilience as a factor that neutralises exposures to risk), the challenge model (a risk factor can enhance adaptation), and the protective factor model (interaction between protection and risk factors reduces the probability of a negative outcome [76]. Further, many of the studies included terms like hardiness, grit, mental toughness as equivalent terms for resilient performance, and also as factors that underpin resilience. The lack of theoretical foundation, and the complexity around resilience, has not prevented the appearance of tools designed to measure resilience in military samples.
Measures and metrics used to study factors associated with resilient performance in defence and security settings. Measures of Performance. There was a large degree of heterogeneity in measures used to assess performance in the included articles. Type of performance measure was categorised into four groups, these include: 1. Selection, Graduation, Completion, which assess participants based on a selection process (typically pass or fail).
2. Applied, including ecologically valid measures conducted 'in the field' such as land navigation or simulated captivity.
3. Shooting, usually conducted in a controlled environment collating performance scores.
Although an ecologically valid measure of military performance, shooting performance was considered separately from those studies in the 'Applied' category.
4. Cognitive, including objective measures in a controlled setting such as sustained attention, memory or visual vigilance.
Studies identified in the review have been found to use one or more of these performance outcomes as outlined in Table 1.
Measures of resilience. Measures of resilience, which were included in the articles that met the inclusion criteria, were categorised into a) psychological measures, comprising subjective self-reports, and b) physiological measures, comprising objective biological measures. Measures utilised in all studies included in the review have been identified and briefly described alongside study characteristics in Table 1.
Resilient performance in selection, graduation, completion (SGC) tasks. Nine of the included studies assessed resilient performance in SGC tasks (see Table 1 for classification). Of these, five studies focused on selection and qualification courses. From these studies there is good evidence that mental toughness is important for successful completion of SGC tasks, and some evidence for the contribution of confidence, resilience (which authors define as a psychological construct) and a stress-is-enhancing mindset. Psychological hardiness may also be relevant, but the estimated effect size was small to medium on performance in these tasks, meaning that larger sample sizes are required to detect significant effects. Further, it may be the commitment facet of psychological hardiness that is specifically important for resilience in these performance paradigms that span a period of weeks. There is currently no consistent evidence for biological markers of resilience in SGC-type performance tasks. The use of similar constructs as predictors of resilient performance is noteworthy. Measures of resilience, grit, mental toughness and hardiness were found in the sample of literature reviewed. One difficulty is that while resilience, grit, mental toughness and hardiness are proposed as different constructs, and each have unique measures, they may not be distinct. Indeed, a recent study testing content, construct and criterion validity of resilience, grit, hardiness, and mental toughness found substantial overlap among the four constructs [77]. Further, measures may not be sufficiently nuanced to provide a detailed overview of the processes in resilient performance. For example the Dispositional Resilience Scale-15 (DRS15; Bartone [78] measures hardy attitudes but does not include the hardiness process (i.e., hardy coping, hardy social support and hardy self-care; [79]. Resilient performance in applied tasks. The stress-performance paradigms that were ecologically valid and conducted 'in the field' were grouped into the 'Applied' task category. Thirteen studies identified were categorised as Applied, of which nine utilised a captivity task (See Table 1).
Five studies measured biological markers of psychological resilience during captivity-type tasks. Markers included saliva cortisol and DHEA-S [68]. Overall performance during the high-intensity challenge was inversely related to the cortisol-DHEA-S ratio, whereas, performance during the low-intensity challenge was related to DHEA-S measured during the captivity phase. These findings are in contrast to other studies [63] that reported that the cortisol-DHEA-S ratio was significantly and positively associated with performance during the interrogation task. Lastly, measures of neuropeptide Y (NPY) [62] were associated with cortisol measures and interrogation performance. Those participants who had a larger cortisol and NPY response to interrogation stress, performed better during the interrogation. One final study did explore adrenal stress and physical performance during military survival training [67], however this did not include a measure of performance for the military task only a handgrip and vertical jump test.
Studies [33] also investigated the relationship between vagal tone (activity of the vagus nerve proposed as an index of emotion regulation and cognitive ability) and performance (captivity phase of the survival evasion resistance and escape (SERE) course and an Underwater Warfare Operations Combat Diver Qualification course). In all of these studies, reduced vagal tone was predictive of superior performance.
Three studies in the applied category assessed performance of military personnel in several physically and psychologically demanding tasks, which included, for example, timed runs, loaded marches, obstacle courses and land navigation ( Table 1). The findings indicate that a stress-is-enhancing mindset was related to faster completion of the performance task, whereas a failure-is-enhancing mindset was related to a slower completion time than average [66]. Farina et al., [47] followed a large sample of 800 military personnel enrolled on the Special Forces Assessment and Selection (SFAS) course. Psychological (Intelligence quotient, Aptitude, Grit, Resilience) and biological (Cortisol, DHEA-S, testosterone, sex-hormone binding globulin (SHBG), and c-reactive protein (CRP)) markers of resilience were weakly correlated with the performance measures. Specifically, increases in DHEA-S were associated with poorer performance. Testosterone, adjusted for SHBG and adrenaline, were related to slower road march times while SHBG, adjusted for testosterone, was positively associated with performance on a number of outcomes. Lastly, increased noradrenaline was related with poorer performance. Biomarkers of stress and immune function were also investigated for their association with performance in 219 members of the Canadian Armed Forces [70]. Levels of cortisol, CRP, adiponectin, INF-γ, TNF-α, IL-1β, IL-2, IL-6, IL-8 and IL-18 were assessed alongside grip strength, aerobic capacity and performance on six military physical performance tests (sandbag fortification, escape to cover, picking and digging, picket and wire carry, stretcher carry and vehicle extrication). Higher CRP was associated with lower performance scores, while there was a positive association between IL-2 and grip strength, and adiponectin was positively associated with plank time while negatively associated with grip strength. An important note here is that collectively these associations were all weak effects, and the multiple statistical analyses that were conducted increasing the chances of a false-positive result.
It is difficult to draw firm conclusions regarding markers of resilient performance in applied tasks given the range of settings, participants and methods used. Dissociative experiences consistently predict poorer performance in captivity tasks, and there is mixed evidence for the predictive value of cortisol, DHEA-S and NPY. Vagal reactivity also significantly predicted performance in captivity interrogation and in an underwater night-time navigation task several weeks after assessment, and therefore might be a useful marker of resilient performance. Other salivary and blood-based biomarkers produced varied predictive power of performance in these tasks. However, methodological limitations, such as small sample sizes and multiple statistical tests, could potentially explain these mixed findings.
Resilient performance in shooting tasks. Four studies assessed resilient performance in shooting tasks (See Table 1). One study explored the relationship between proinflammatory cytokine markers (TNF-a, IL-10), Brain-Derived Neurotrophic Factor (BDNF) and shooting performance in the elite combat unit of the Israel Defence Force [28]. Increases in inflammatory response were associated with lower levels of shooting performance; however, there was no relationship between the key marker of resilience, BDNF, and performance. The effect of sustained physical and psychological demand on marksmanship was explored during 'hell week' of Navy SEAL training [57]. Marksmanship was impaired during 'hell week' on a number of variables (e.g., distance from the centre of mass, shot group tightness, sighting time and number of missed targets). Performance on a range of cognitive variables (e.g., reaction time and accuracy on cognitive tasks) were worse, and mood was more negative, during 'hell week' compared to baseline. However, there was no analysis of how changes in key variables that might be indicative of greater resilience (e.g., more positive mood state) related to marksmanship performance.
Marksmanship performance in psychological demanding environments was the basis for a very well-controlled study of police officers [56]. Participants took part in shooting tasks in both high stress (24 trials) and low stress (24 trials) conditions (counterbalanced). Shooting accuracy was greater in the low stress compared with high-stress condition. Further analysis indicated officers with experience, high levels of thrill and adventure seeking, and low levels of behavioural inhibition performed better in the high stress setting, with greater experience being the strongest predictor.
The final study [45] explored how training programmes might be beneficial for shooting performance. Performance time on a shooting task was assessed in relation to participation in a training programme (Tactical Human Optimisation Rapid Rehabilitation and Reconditioning program; THOR3) that has aspects of psychological training. There was no statistically significant difference in performance between the participants who had undertaken the THOR3 programme and those who had not on performance time. Despite the difference not being statistically significant there was a small to medium effect for the difference between the groups, with better performance in the group who had undergone the THOR3 programme.
It is difficult to draw meaningful conclusions from four studies. However, the data does indicate that even among trained performers (e.g., Navy SEAL recruits and police offers) shooting performance under physical and psychological stress is impaired. From a resilience perspective, manipulating psychological stress and building on the work of Landman, Nieuwenhuys [56] shows promise. Introducing psychological stressors that draw on previous research such as uncertainty, or danger such as meaningful evaluation (psychological) or the potential for pain (physical), and effort (difficult task) will place participants under stress and enable exploration of resilience variables which may predict performance. It was also noteworthy that aside from the Landman et al. study, there was no clear assessment of psychological and physiological state close to the task itself and while doing so presents challenges, this will give a clearer indication of what measures are important in the resilience-performance link.
Resilient performance in cognitive tasks. There were eight studies that collected data on cognitive performance (See Table 1). Four studies explored solely how cognitive performance deteriorated under psychological and physical stress [57][58][59]65], and four studies explored how markers of resilience may relate to cognitive performance [28,33,71,72].
Studies that explored cognitive performance deterioration used a range of computer based cognitive tasks following various psychologically demanding and stressful tasks including 'hell week' during NAVY seal training [57], a cold stress task [58], and SERE US Army training course [59]. All three studies indicated that the stress task or condition was associated with decreased cognitive performance. Różański, Jówko [65] explored the association between biochemical markers of stress and a divided attention cognitive task in male special force soldiers. There was an increase in performance on the divided attention task across a survival training programme. Further, while a trial test of the divided attention task was performed, there was no evidence that performance had plateaued and the increase in performance seen, despite the psychological and physical stress of the 48-hour exercise, may be representative of learning effects on this specific task. Collectively, while biomarkers of physical stress were taken in some, there was no exploration of how these variables changed in relation to performance on the cognitive task. As such, beyond concluding that the physical and psychological stress of a demanding environment has an impact on some aspects of cognitive performance, it is not possible to elucidate the resilience-performance link from the data.
Another four studies explored indicators of resilience and the link with cognitive function. Increases in heart rate [72] and BDNF [28] were associated with better cognitive function, suggesting promise for both as a marker of cognitive performance after a prolonged period of psychological and physical demand. However in other studies [71] serotonin levels were unrelated to cognitive performance and although vagal tone as explained previously predicted performance in an applied test and Navy SEAL selection, it was not associated with performance on a written test [33].
The data on cognitive performance reports relatively few studies that explicitly link markers of resilience with cognitive functioning. In the study by Yao, Yuan [72] it was interesting that a greater increase in heart rate was associated with better performance after errors during a task. That is, a stronger stress response (assessed by heart rate) was associated with better cognitive function, and is proposed to represent greater effort and mobilisation of cognitive resources to atone after the error. The finding that a stronger stress response may be associated with better recovery after an error is a timely reminder that a strong stress response is not, in and of itself, negative, and may have positive performance effects [80]. Similarly, given the sample of elite military performers, the data suggesting that following a period of psychological and physical demand (5-day training programme) BDNF concentration may be a promising biomarker of potential cognitive performance was also noteworthy [28].
Programmes designed to bolster resilient performance in defence and security settings. Surprisingly few programmes specifically described seeking to improve resilient performance in high-performance defence and security settings. Four studies explored cognitive performance in the context of training programmes [40,53,60]. There were, additionally, two studies that specifically utilised mental skills training to improve performance [29,32].
Three studies explored cognitive function in the context of mindfulness training programmes [54,60] and one using an attention training programme [40]. Jha and colleagues ran a series of studies aiming to promote cognitive resilience using mindfulness to protect against the degradation of attention over time. In two separate studies performance remained more stable for those who received in class mindfulness training [53] and those who received training from Master Resilience Trainer [54] on a Sustained Attention to Response Task (SART). The study by Meland, Ishimatsu [60] explored mindfulness training with Norwegian military helicopter units during their preparation for redeployment to an aeromedical mission in a conflict area. Those who received mindfulness training reported lower perceived mental workload on the SART. However, there was no difference in performance accuracy and indeed the mindfulness training group reported significantly slower reaction times.
A range of other training programmes have also been investigated to increase resilient performance. This includes the study of training for Adaptability and Resilience in Decision-Making (STAR-DM) approach [40], in which groups of squad leaders who received the training were compared to control group squad leaders on their ability to make scenario based decisions. The small sample size (n = 13) means it is difficult to draw any firm conclusions, but there was tentative evidence that the STAR-DM training packages can improve decision-making during training. Another programme also included general mental skills training (MST; Jensen et al., [29], investigating the cognitive skills and performance across a 12-week, three stage, training course. Soldiers were randomly assigned to one of three specific groups embedded into the course: a training-as-usual group (n = 91, given free time during the structured MST instruction), general mental skills training, covering goal setting, arousal control, imagery, positive self-talk and focus/concentration (n = 47), and mindfulness-based mind fitness training (n = 65). Performance in both mental skills groups was better than the training-asusual group during the phase one hike and phase three hike, although performance of the training-as-usual group was higher during the cognitively demanding communications test in phase 3, than either mental training group. Overall performance of the training-as-usual group was significantly the highest during phase 1, while performance of the general mental skills training group was significantly the highest during phase 2. Finally, while, cortisol levels were lowest in the mental training group pre-ambush, similar levels were seen across all three groups during the ambush and after the ambush itself. In sum, there is some evidence for mental training programmes enhancing cognitive performance and altering stress response prior to a stressful task, but the effects on actual performance were less clear.
A similar mental skills intervention was conducted by Fitzwater, Arthur [32] in male British Army Parachute Regiment recruits (P-Company) taking part in a training course. Five platoons were exposed to a psychological skills training program comprising goal setting, relaxation and arousal regulation, self-talk and imagery, while five platoons in the control condition did not receive any exposure to psychological skills training. There was a significant increase in the observer-rated measure of mental toughness in the intervention group and no change in the control group. In addition, individual performance was significantly higher during P-Company for the intervention group when controlling for fitness and leadership climate. In sum, the three-week intervention showed some positive effects, both in terms of changes in mental toughness and performance.
There is a paucity of research using interventions to improve performance resilience in defence and security settings. To date, focus has been on mindfulness or general psychological skills interventions. Both interventions show some promise, with effects generally clearer on cognitive tasks rather than in real world performance. The study by Fitzwater, Arthur [32] is noteworthy in that regard, with a clear measure of performance based on relevant tasks, and a clear focus on the potential mechanisms (e.g., psychological skills usage) while controlling for potential confounders (e.g., fitness level, leadership climate).
Development of resilient performance framework. To stimulate future work on resilient performance in defence and security settings, a theoretically informed organisation of factors, integrating findings of the present review alongside principles identified in the broader resilient performance literature, is identified in Table 2.
Due to the diversity of research identified in the review, this is a logical and critical step for synthesising the existing literature around an agreed definition and understanding of what resilient performance is. As previously outlined in this review, resilient performance is defined as: The maintained or improved execution of competence under situational duress. The proposed framework includes three levels: global-contextual enablers, situational processes, and markers of resilient performance. Global-contextual enablers refer to the stable dispositional, trait and ability-like variables that were studied in a number of the reviewed articles (e.g., hardiness, mental toughness). These factors are referred to as 'resilient performance enablers', which is consistent with recent progress in the area of thriving under pressure [23]. In the broader literature researchers have used the term 'resilience factors' [81,82] to describe these same variables. However, this terminology is somewhat ambiguous given 'factors' is often used interchangeably with the term 'variables' to discuss any measured phenomenon. Stable resilient performance enablers are predicted to influence the situational stress response and onward resilient performance through situational 'resilient performance processes'. Such processes include physiological dynamics and psychosocial resources and appraisals that are activated in response to specific stressful demands.
Preliminary evidence identified in the review has implicated such processes in resilient performance, which is again consistent with the broader literature on stress, resilience and performance. As indicated earlier, although a number of key resilient performance processes were identified in the review, additional process variables (e.g., social connectedness) would also be expected to play an important role [27]. At the highest level are the specific markers of resilient performance. In the review, a number of indicators of performance relevant to defence and security were identified, including marksmanship, attentional focus, and land navigation, amongst others. Broadly, situational indicators of resilient performance (i.e., those factors that indicate the likelihood of an individual being able to successfully complete a discrete task under conditions of adversity) can be considered along physical, technical, cognitive affective, and team lines. The specific variables that are indicative of resilient performance will be, in part, dictated by the tasks being completed. For instance, resilient performance during a direct-action mission will require performance across a range of skills that may be different to those required for completion of a reconnaissance operation [22]. Acknowledging the complexity of the human experience, resilient performance markers are expected to be networked in the way that Kalisch, Cramer [27] suggest, and so can exert interacting effects upon one another (e.g., physical performance can affect technical performance) and be closely coupled with the resilient performance processes. Experiences of resilient performance are likely to have a recursive relationship with psychological resources such as perceptions of control and self-efficacy. These recursive interactions where processes impact performance and vice versa, are expected to continue for the duration of stress exposure and may fluctuate depending on the changing nature of stressful demands.

Discussion
The aim of the current review was to examine underpinning theories and models, measures and metrics, and approaches that could be used to maintain and train resilient performance in defence and security personnel. Overall, many of the studies lacked a clear theoretical underpinning. A variety of measures and metrics from self-report measures through to biomarkers were used to assess the associated antecedent factors that might be indicative of resilient performance. In that regard, these findings are similar to those of a recent meta-analysis on the link between psychological resilience and mental health and functioning in military personnel, where the heterogeneity of the measures of resilience, and health and functioning outcomes hindered clear conclusions [13]. The few training studies that were identified in the review focused on mental skills, mindfulness, and resilient decision-making, which might be targets for enhancing resilient performance. The following discussion is an attempt to integrate the findings of the review alongside more general progress related to the study of resilient performance as it pertains to defence and security settings. In general, there was limited use of coherent theoretical perspectives to rationalise the purpose of the studies or the measurement practices undertaken and then discuss the study findings. Several of the articles did focus on well-established psychological principles (e.g., mental skills), however, these principles were typically not discussed or integrated into broader explanatory frameworks (resilience frameworks or otherwise). The lack of theory is problematic, especially when there are unexpected findings, like those that emerged in several of the reviewed papers (e.g., decreased vagal tone linked to optimal performance) [33]. To build on existing research and make progress in this area, there needs to be clarity around how resilience is defined, conceptualised and the expected relationships between important antecedents, processes and outcomes. A number of studies included in the present review treated resilience (explicitly or implicitly) as a trait-like variable. Although there continues to be debate about the nature of resilience [83], contemporary definitions and models of resilient performance specifically tend to view the construct more as a process of person-environment interactions, which can be influenced, but not directly represented, by underlying traits and dispositions [81]. The present work is guided by this latter view and the definition put forth by Adler, Williams [16] who suggested that resilience is the positive adaptation to adversity or stress. This positive adaption will support sufficient performance levels to maintain important competencies, like those identified by Pulakos, Arad [1], and achieve a successful outcome.
Future work in this area should draw on the resilience theory portrayed in contemporary general and performance psychology literature to provide sound studies of theory based on evidence-based expectations and measurement practices.
Unlike some other high-performance domains, exploring the resilience-performance association in defence and security settings is complex because performance can encompass such a wide range of tasks. For example, performance might include completing a physically demanding assault course and resisting interrogation in a prisoner of war scenario, through to conducting covert reconnaissance. The qualities needed to perform in these, and other, scenarios will be different., The dynamic resilience network approach [27], can be used to acknowledge these unique contextual features. As the model outlines psychological resilience as a variable construct, that is context specific and dependent on person-environment interactions. According to this perspective, resilience is best understood by the dynamics in the interconnections between physical, affective, cognitive and social nodes. Therefore, this is a useful starting point from which to develop an understanding of the resilience-performance relationship in defence and security settings. This framework is used to outline resilient performance enablers, resilient performance processes, and markers of resilient performance, and these are discussed below.
A number of resilient performance enablers, or trait-like resilience factors, were identified in the literature review. This included, amongst other variables, hardiness, grit and mental toughness. These enablers are expected to contribute to adaptive processes under conditions of stress and pressure, thus increasing the likelihood of resilient performances. There is evidence that these types of responses are likely to contribute to effective performances [84]. However, the current review identified unique enabling factors. There is an ongoing debate about the extent to which factors like hardiness, grit and mental toughness are divisible, or indeed whether they are all part of the same psychological construct [85]. Delineating enablers that have incremental predictive validity on resilient performance outcomes seems to be a worthwhile future pursuit. Looking beyond the current review, there are a range of other potential resilient performance enablers that might be of interest. Potential enablers include intelligence, personality, motivation and general perceptions of social support [20].
A variety of situational processes of resilience were identified in the literature. There was consistent evidence across specialist groups and general soldiers that during interrogation a higher level of self-reported dissociative symptoms was associated with poorer performance [46,62,69]. In contrast, there was no reliable, consistent evidence of biomarkers being associated with performance across a range of performance indicators. During captivity-type tasks reduced vagal tone was predictive of superior performance in the captivity phase of the SERE course [33]. In the same paper Morgan et al. reported that as with performance in the SERE course, reduced vagal tone was associated with improved performance in the underwater navigation task several weeks later. Other salivary and blood-based biomarkers (e.g., cortisol, adrenaline and DHEA-S) produced varied predictive power of performance in these tasks, although methodological limitations, such as small sample sizes and multiple statistical tests, could potentially explain these mixed findings. Outside of the military, recent studies examining cortisol and DHEA in performance settings indicate a relatively small contribution [24]. When the performance tasks were specifically focused on marksmanship or cognitive performance the results were less clear. One promising process biomarker is BDNF concentration [28] which was positively related to cognitive performance in an elite military sample.
From a process perspective, it was notable that there was limited consideration of social factors, specifically leadership and team factors (e.g., cohesion), in relation to performance. While leadership style was considered as a covariate by Fitzwater, Arthur [32], it did not feature in the remaining studies. There is consistent evidence that leaders, and leadership style, may impact stress responses [86]. Further, social support has been consistently associated with positive stress responses and better performance under stress (see Meijen, Turner [26], for an overview). Satiation of relatedness needs (linked to leadership and social support) has also recently been associated with experiences of thriving under pressure [24]. Cohesive groups can help provide the support needed to fulfil social connection needs and in the process contribute to resilient function. A novel biomarker that may also help indicate the strength of social connection within a group and the potential buffering against stress is Oxytocin (a neuropeptide produced in the hypothalamus). Oxytocin plays an important role in prosocial behaviours [87] and is associated with lower levels of cortisol under acute stress [88][89][90][91], particularly in tasks that are sufficiently stressful to elicit a strong HPA axis response [92]. Further, oxytocin may be an important factor in determining a challenge state, which is considered to be an adaptive response to stress that is indicative of positive coping resources, in contrast to a threat state which reflects insufficient coping resources [26]. Meijen, Turner [26] also proposed that NPY might be associated with a challenge state, with peripheral plasma NPY (which was assessed in the military studies by Morgan and colleagues) having an effect of decreasing hypothalamic pituitary-adrenal (HPA) axis activation [93]. These overlapping processes are interesting and may point towards a collection of biomarkers that are directly implicated in the stress-resilience-performance relationship and should be explored in future work.
While there are some promising findings, there is no single process marker (biological, psychological or social) that has been consistently found to predict performance of defence and security personnel. This is not particularly surprising given that performance demands may vary across different defence and security populations. There are also methodological and practical challenges associated with collecting data from the number of personnel at the right times to be able to conduct robust and adequately powered studies. It could also be argued that the 'resilience' required to perform a physically demanding exercise is different to the 'resilience' required to work effectively as part of a team in a demanding environment. In the literature review, a range of performance contexts were identified: pass/fail, selection or a course, physical performance, applied performance, cognitive performance, marksmanship. These provide a useful starting point, but perhaps are not entirely inclusive of the range of competencies that may be relevant to the work of defence and security personnel, see Pulakos, Arad [1]. Future studies may conduct a more rigorous task analysis to capture the different physical, technical, cognitive and affective, and social dimensions of performance. It would be possible to examine in a systematic way, which processes (including biomarkers) are most predictive of performance in specific tasks.
One strand of research, from other performance domains, which may have utility in explaining a broad range of relevant performance outcomes is research around challenge and threat states. In addition to work highlighting the importance of situational challenge appraisals [20,21] there is a consistent body of work demonstrating different patterns of cardiac reactivity related to challenge and threat states [94,95]. A challenge state facilitates improved decision-making, effective and maintained cognitive function, decreased likelihood of reinvestment (which affects skills motor performance), efficient self-regulation, and increased anaerobic power; all of which are likely to lead to successful performance [25]. In contrast, a threat state leads to ineffective decision-making and cognitive function, increased likelihood of reinvestment (which impairs skilled performance), inefficient self-regulation, and decreased anaerobic power (compared to a challenge state); all of which are likely to lead to unsuccessful performance (Jones et al., 2009 [25]). In short, in a challenge state, Sympathetic-Adreno-Medullary (SAM) activation is fast-acting and represents the mobilisation of energy for action (fight or flight) and coping. A threat state accompanies slow-acting Pituitary-Adreno-Cortical (PAC) activity, (and SAM) activation and represents a "distress system" associated with perceptions of actual harm [15]. As an underpinning philosophy, challenge and threat holds that stress itself is not deleterious for performance, rather, it is whether stress is characterised by challenge or threat that is important for performance. Building on this work related to cardiac responses under stress and merging it with some of the work on vagal tone may be a promising avenue.
The current review has wide implications for the development of resilient performance training. The resilient performance framework in Table 2 provides a context within which to understand how resilient-focused training programmes could be designed to develop and maintain or improve performance. Resilient-focused training programmes that were identified and reviewed were developed with a focus on mental skills, mindfulness and resilient decisionmaking. Given the small number of studies, caution is needed when interpreting and generalising the findings. However, there is initial evidence that training in some of these areas has a small-to-moderate effect on performance. Findings highlight why mental skills training, for example, might be the focus of emerging military research [96].
The link between mental skills and performance in the present work is consistent with the broader literature on the benefits of mental skills training in other performance settings, such as high-level sport [97]. The ability to use such skills appears beneficial for sustained performance under stress, yet the mechanisms of effect remain to be well-explored. This is especially true in defence and security contexts. Borrowing from theoretical frameworks related to stress self-regulation, there may be a dynamic component to the utilisation of these mental skills, which relies on an individual's capacity to accurately evaluate and appraise the demands of the context, select an effective strategy (i.e., a mental skill) and monitor the impact of that strategy on their performance, adjusting the approach as necessary [98]. It may be that this flexibility in the deployment of mental skills is equally, if not more, important than simply knowing about the skills themselves. This type of model provides the link between the knowledge and ability to use mental skills and the biopsychosocial processes that may be proximal determinants of resilient performance outcomes. Addressing these types of unknowns is important for developing the most effective training (mental skills or otherwise) for defence and security personnel.
Overall, based on the evidence that is available, training resilient performance in defence and security is likely to require a focus on multiple areas. The reviewed literature primarily emphasises targeting enablers (e.g., mental skills, mindfulness). However, earlier discussed correlational findings and results from beyond military populations also suggest that intervening at the process level, for instance by modifying resource appraisals and optimising stressorstrategy coupling, could reap performance benefits. In its delivery, training that includes educational, modelling and applied components should empower, coach and equip individuals and teams with the information and skills needed to foster resilient performance under conditions of stress and pressure. This training would potentially be most valuable when it is compatible and could be built into day-to-day activities such as marksmanship training, enabling ongoing opportunities for modelling and application, which may be beneficial for maintenance [96].

Future directions
This is a growing area of research, which the current review has attempted to synthesise despite the lack of homogeneity in published articles. Of note is diversity in the sample, including papers that failed to meet the inclusion criteria of the current review (e.g., participants aged under 18, [99]. Additionally, a range of papers that did not formally measure performance outcomes yet the construct was part of the research context (for example Eid and Morgan [46]. Later work could seek to extend the scope of the current review to encompass a range of study types within VUCA settings. Several other suggestions for future work have been provided throughout the discussion, two of which are particularly important for the research topic. First, is the development of a valid and reliable measure (or collection of measures) of resilient performance in defence and security settings. Conducting rigorous, theoretically informed research, requires tools that can be used to accurately examine variables of interest. Given the organisation of factors in Table 2, any such tool designed to capture resilient performance in these settings is likely to need a dispositional and situational component and may incorporate various types of assessment method. In addition to measure development, systematically analysing the strength and direction of associations between enablers, processes and markers of performance over time is also a critical step. Approaching this work with clear hypotheses and conducting studies in an iterative fashion, would contribute to advancing understanding and lay the foundation for a biopsychosocial theory of resilient performance specific to defence and security personnel.

Conclusions
Findings of the review suggest that although established psychological concepts were often discussed, studies were largely lacking in underpinning explanatory theory (resilience theories or otherwise). Furthermore, a range of performance-related measures were pinpointed, which may provide the basis for examining networks of resilient performance in future defence and security specific studies. In addition to performance markers, a variety of global/contextual resilient performance enablers and situational resilient performance processes were identified. These variables offer some indication as to the antecedents of resilient performance and potentially suggest a target for future resilient performance training. From a performance standpoint, there were some notable omissions in the literature. In particular, few studies examined the role of social factors in performance, despite this being a critical component in high performing teams. Existing training programmes were narrowly focused on mental skills, mindfulness and resilient decision making.